1. Field of the Invention
The present invention relates generally to a propeller blade for a wind turbine generator (in the following simply referred to as “windmill blade”) and a method of making such a windmill blade. The windmill blade comprises reinforcing fibers in a cured and toughened epoxy resin matrix.
2. Discussion of Background Information
Epoxy resins are frequently used for the production of fiber-reinforced blades of windmill propellers by a process which comprises infusing a liquid epoxy resin composition in a fiber reinforcement and thereafter curing the composition. The propellers of a wind turbine generator usually comprise three individual blades having a length of typically at least 20 meters and often up to about 40 meters or even longer (e.g., up to about 70 meters). Due to the length of the propeller blades the forces these blades are subjected to especially when they are in use (i.e., when the propeller turns) are substantial, making it desirable or even necessary to toughen the matrix epoxy resin by adding a toughening agent to the matrix resin composition.
There are many types of toughening agents for epoxy resins but all of them have a tendency to create processing issues. Specifically, reactive liquid polymers tend to have a high viscosity which means that the infusion process must be modified. For example, U.S. Patent Application Publication 2004/0053055 A1, the entire disclosure of which is incorporated by reference herein, teaches the use of liquid toughening agents but makes it clear that the resultant compositions must be heated in order to exhibit a viscosity which is suitable for processing. For example, section 0022 thereof states “The curable compositions can be used at temperature above about 100° F., or even over about 110° F. Typical operating temperatures are around about 120° F. At the above mentioned operating temperatures, the curable compositions have viscosities below about 450, or below about 400, or below about 350 [cps].” Further, obtaining these relatively low viscosities requires not just the application of a temperature above ambient temperature but also the addition of a low viscosity reactive diluent to help further reduce the viscosity of the composition which is to be infused in the reinforcing fibers.
Preformed particles such as core-shell rubbers or even inorganic particles can also be used to toughen epoxy resins. See, for example, Sprenger S. et al., “Rubber toughened FRCs optimized by nanoparticles”, JEC-Composites, No. 19, August-September 2005, pp. 73-76, the entire disclosure of which is incorporated by reference herein. These particles require dispersion into an epoxy resin first, which results in viscous epoxy resins. While the final formulation can be diluted with reactive diluents to help reduce the viscosity, too much reactive diluent can lead to a reduction in mechanical properties of the article molded therefrom. Additionally, there is a chance that some of these preformed particles are filtered out of the liquid formulation during the infusion process by the fiber matrix which is a key component of the final composite blade. This can lead to a reduction in the toughness of at least parts of the composite because less toughening agent will be present in the entire thermoset system or at least parts thereof.